Quarkonium polarization in heavy ion collisions as a possible signature of the quark-gluon plasma

The polarization of quarkonium states produced in hadron collisions exhibits strong nonperturbative effects - for example, at small transverse momentum p_t charmonia appear unpolarized, in sharp contradiction to the predictions of perturbation theory. The quark-gluon plasma is expected to screen away the nonperturbative physics; therefore those quarkonia which escape from the plasma should possess polarization as predicted by perturbative QCD. We estimate the expected J/ψ polarization at small p_t, and find that it translates into the asymmetry of the e⁺e ^-(μ⁺/μ^-) angular distribution W(θ) ∼ 1+αcos²θ, with α≃0.35-0.4.